Lettuce (Lactuca sativa L.) originates from Mediterranean regions where the summers are long, hot, and dry. Lettuce seedlings have the highest survival prospects when emerging from seeds in early spring. To avoid premature germination during summer, lettuce seeds are equipped with a secondary (or induced) dormancy mechanism called thermo-inhibition. During the heat of summer, thermo-inhibition prevents seeds from germinating during a short period of auspicious circumstances, such as a single rain-shower. Only during the relative long cold of winter, thermo-inhibition is relieved, thus enabling germination to occur when temperatures rise in early spring.
During the domestication of lettuce, thermo-inhibition was maintained. Most commercial lettuce seeds are inhibited from germinating at high temperatures, the exact temperature is dependent on the cultivar and the seed production environment. This means that modern lettuce cultivars, when sown in hot conditions, will go into dormancy. This is especially problematic for winter lettuce growing areas, where permissive germination temperatures are exceeded during the late summer and autumn planting season.
The optimal germination temperature for the majority of lettuce varieties lies between 15° C. to 22° C. To obtain lettuce seeds capable of germinating at temperatures well above 22° C., seeds are subjected to a rather expensive germination stimulating process called “priming”. Seed priming allows for the controlled hydration of seeds, allowing the seeds to complete the first steps in the germination process before they are dried back to their original moisture content, and stored until planting. One of the primary benefits of seed priming is the ability to alleviate thermo-inhibition by increasing the maximum temperature at which germination will occur.
Although seed priming may be beneficial to seed germination, it is an expensive procedure in terms of labour and equipment requirements, the types of ingredients that are used, and the time it requires for hydrating and drying back the seeds. In addition, the priming process may result in a reduction of the shelf life of primed seeds, as compared to untreated seeds. This undesirable side effect is influenced by the rate and extent of the drying back procedure.
Moreover, there is an inherent risk of “overpriming” which may lead to damage of the radical tips of the seeds, and subsequently, poor seedling growth. Overpriming would render primed seeds useless, thus making elaborate seed quality checks an additional necessity of the priming process.
By developing seeds of the species Lactuca sativa L. that are capable of germinating at high temperatures without the need for priming, the expensive and potentially precarious process of seed priming becomes obsolete.
Improving the capability of lettuce seeds to germinate at a high temperature may also enlarge the total acreage for lettuce cultivation. Areas of the world with relatively warm winters are unsuitable for lettuce cultivation, since the germination capabilities of current lettuce varieties are insufficient to overcome thermo-inhibition under such high temperatures.
In a more global context, rising temperatures due to global warming may have a considerable impact on soil temperature. As such, high temperature, and the resulting increase in soil temperatures, is considered a significant environmental stress that may limit worldwide crop productivity in the near future.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.